Previous studies have shown that the alga Chlorella synthesizes homocysteine predominantly be transsulfuration, and no activity of direct sulfhydration could be detected. These studies have been extended to the higher plant Lemna, which has been shown to synthesize homocysteine also predominantly by transsulfuration. However, in the case of Lemna it was convincingly demonstrated that direct sulfhydration did contribute a small proportion (in the range of 5-10%) of the total homocysteine biosynthesized. Lemna has been selected for long-term studies on the mechanism of control of methionine biosynthesis in the two pathways. Conditions for optimal growth of Lemna, and the limits within which these conditions must be controlled, have been determied. Lemna was found to exert a remarkable control over sulfur metabolism which permits both the pool sizes of sulfur amino acid intermediates and the synthesis of organic sulfur compounds to remain essentially unchanged when the plant is presented with a 3000-fold variation in the environmental concentration of inorganic sulfate. Plants growing in the presence of methionine can increase the size of their soluble methionine pool over 50-fold, where it now represents a significant fraction of the total cellular methionine. BIBLIOGRAPHIC REFERENCES: Datko, A.H., Mudd, S.H., and Giovanelli, J.: Homocysteine biosynthesis in green plants: studies of the homocysteine-forming sulfhydrylase. J. Biol. Chem. 252: 3436-3445, 1977.